Conventionally, the lashing of reinforcing bars to form reinforcing mats is carried out with the aid of simple, manual tools, which is a very time-consuming and thus expensive and also laborious operation, which is apt to give rise to attritional injuries. The reason for this is that, when lashing reinforcing mats for concrete slabs, arches or the like using previous aids, the operator has to bend for long periods of time, which leads to great stress on the back.
In this connection, the lashing of the reinforcing bars is usually effected with the aid of tongs or a twisting tool, by means of which the ends of a wire, which is arranged by hand around the reinforcing bars at the various intersection points, are twined or twisted together for firm connection of the bars. Carrying out lashing in the conventional manner also involves risks of accident, especially in the case of work on roofs, bridges and the like, owing to the stooped working position which, inter alia, leads to risks of falling accidents.
In my International patent application PCT/SE91/00571, a machine is described for binding intersecting bars together by means of wires, in particular for lashing reinforcing bars, which machine works in accordance with the method indicated in the first paragraph above. By means of this machine, the lashing operation can be made considerably more efficient at the same time as the abovementioned risks of injury can be eliminated or considerably reduced, as this machine allows the operator to work upright.
In known lashing machines, which are provided with a rotatable twisting head arranged above the jaws so as to replace the manual twisting-together of the wire ends with the aid of tongs or the like, there is inter alia a risk of the lashing wire breaking as a result of excessive tension in it if the wire parts are secured in the twisting head. Furthermore, it can be difficult, owing to tensile forces in the wire, to make the latter surround the reinforcing bars closely at the twisting location.
The present invention is based on the knowledge that this is due to the fact that the machine cannot imitate the movement of the hand during manual twisting. When manual twisting-together of two wire ends is carried out, the wire is first stretched around the reinforcing bars. During twisting, the hand and thus the wire ends will then approach the upper reinforcing bar in the course of twisting owing to the fact that the twisting itself requires a certain wire length.
In a twisting machine with a rotatable twisting head, the wire ends are usually secured in the twisting head throughout the twisting operation, which means that the extra wire length required for the twisting itself cannot be fed down through the twisting head. This results in increased tension in the wire, which may lead to the latter breaking and those portions of the wires which are twisted together being drawn up from the reinforcing bars, so that an interspace is formed between the uppermost bar and the twisted-together portions of the wire ends.
Another disadvantage of known lashing machines is that they comprise pivotable claws which have to be closed and opened a great many times a day by hand movements of the operator, which is very tiring. Moreover, the risk of functional disorders of the machine increases.